I wrote a serial bootloader for STM8S. It’s fast and doesn’t consume much flash (fits in 560 bytes w/SDCC v3.6.8 #9951). I also ported the bootloader protocol to ESP8266, which has various features like drag-n-drop upload, file storage and remote terminal. This project might be useful to the owners of the ESP-14 module, which has an STM8S003 on board.